A transmitter's overall efficiency within a wireless terminal, such as a cellular phone, is a key metric in determining the operating time or talk time for the wireless terminal. The wireless terminal comprises a power amplifier (PA) that consumes a sizeable amount of current when transmitting signals at relatively high output power levels. One method to reduce the PA current consumption and/or improve PA efficiency is to modulate the supply voltage for the PA. For instance, the supply voltage can be modulated using an envelope tracking (ET) system that dynamically adjusts the supply voltage for the PA based on the PA's input signal level. Rather than applying the wireless terminal's battery voltage directly to the PA's supply voltage, the wireless terminal's battery is typically connected to an ET modulator, and the output of the ET modulator is connected to the PA supply voltage pin. The ET modulator can reduce the PA's supply voltage when the PA input signal level is relatively low and increase the PA's supply voltage when the PA input signal level is relatively high. Thus, the ET modulator improves the efficiency in providing the PA's supply voltage by reducing unnecessary headroom.
One of the challenges for implementing an ET system within a wireless terminal is managing the PA load variations seen by the ET modulator. Typically, the ET modulator can experience PA loads that substantially fluctuate depending on the PA's power. For example, when the PA outputs relatively low power, the PA consumes relatively low current causing the impedance on the PA's supply voltage line to be relatively high. Conversely, when the PA outputs relatively high power, the PA consumes relatively high current causing the impedance on the PA's supply voltage line to be relatively low. To accurately track the varying PA loads, a closed loop ET modulator may implement a feedback loop that tracks the PA load and adjusts the PA's supply voltage accordingly. Unfortunately, the feedback loop utilized by the closed loop ET modulator can often cause stability issues that affect the overall performance of the ET system.
As an alternative, an ET system may utilize an open loop ET modulator to mitigate the stability issues that arise from the feedback loop within a closed loop ET modulator. In contrast to a closed loop ET modulator, the open loop ET modulator may not implement a feedback path to detect the varying PA loads. Unfortunately, without a feedback path, the PA load variations may cause the open loop ET modulator to yield varying voltage reference inputs that produce an overall system degradation of the wireless terminal. To alleviate this problem, the open loop ET modulator may need to be configured to digitally pre-distort the PA load variation. However, for the pre-distortion to be effective, the open loop ET modulator may need to obtain accurate information about the imperfections between the open loop ET modulator and the PA's supply. For instance, the open loop ET modulator may need to obtain information about the PA load variations under dynamic signal excitation (e.g. Long Term Evolution (LTE)), as well as the inductor-capacitor (LC) filter response information.